[DGonzo2015]

Liquid water has no definite shape or structure. As you freeze liquid water, the disorganized structure changes one into a definite crystal shape. WHY does this happen? As you boil water, I understand that the heat forces the bonds to break, but I don't understand how cold makes water reform into a definitive crystal shape.  Thanks.

[gritch]

Water is a rather odd molecule in this respect. The oxygen of the water molecule is considerable more electronegative than the two hydrogens (ie. the oxygen holds onto its electrons more tightly than the hydrogen atoms do) leading to a polar bond between the two atoms (there is more electron density towards the oxygen than the hydrogens of the water molecule). This disparity of electrons is so great and the charge of the hydrogen is small enough such that the water molecules engage in what is known as hydrogen bonding between several molecules of water.

Hydrogen bonds are strong interactions between several molecules of water. They help keep molecules of water closer together, making condensed states of the material more stable. The fact that water is a liquid are room temperature at all is rather odd and is due mostly because of these hydrogen bonds - most molecules of a similar size are gases at room temperature (methane, dinitrogen, carbon monoxide).

These hydrogen bonds also lead to another unusual phenomena, the expansion of water as it goes from liquid to solid phase. While relatively strong these hydrogen bonds are able to be broken thermally. At higher temperatures (~100°C) there is very little hydrogen bonding between water molecules, there's just too much energy and they immediately break apart. At around room temperature these interactions become more stable but there is still enough thermal energy to elude crystalline structures. At lower temperatures (~0°C) these hydrogen bonds are stable and form ridge pseudo-covalent bonds between water molecules. The standard morphology of frozen water is a 3-D hexagonal lattice (sort of like a 3-D honeycomb) which actually leads to a less dense material than in the liquid phase.

I think that's everything about the phases of water. Sorry that I went on for a while there.

[Arkcon]

Hmmm ... virtually all pure compounds on Earth can exist as solid or liquid or gas depending on temperature.  I don't really understand what the question is regarding water -- maybe because its possible to see all 3 states in the environment without very expensive equipment.  To the O.P. -- do you wonder why molten iron becomes solid at lower temperatures?  Thermal energy is responsible for overcoming intermolecular bonds and making a solid into a liquid.  There's nothing special about water in this regard.

[opsomath]

OP has actually raised a fascinating question. It's totally weird that water has this inherently ordered structure that magically appears as it gets cold.

The explanation for this is found in the eternal balance between enthalpy and entropy. Essentially, what is going on is that the hexagonally ordered structure of ice is what water wants to do at that pressure. The only way it is going to not do that is if you put a crap-ton of heat into it, making all the molecules vibrate so hard that the hydrogen bonds literally cannot hold them together.

[DrCMS]

a crap-ton of heat

Is that a new SI unit I missed the introduction of?

[curiouscat]

OP has actually raised a fascinating question. It's totally weird that water has this inherently ordered structure that magically appears as it gets cold.

How is this appearance of order when cooled, in general, any more unique than when freezing ethanol or benzene etc?

[opsomath]

How is this appearance of order when cooled, in general, any more unique than when freezing ethanol or benzene etc?

You're right! It's not. But people tend to find about water first because of its unusual property of expansion due to that hexagonal lattice structure.

Is that a new SI unit I missed the introduction of?

I remember when I took quant, we developed an actual numeric conversion between "sh*tload" and "f*ckload". Don't remember what it was, though. Those non-SI unit conversions are tricky and nonintuitive.

[Corribus]

I think you're better off just using kilo-sh*tload or mega-sh*tload instead of trying to do that conversion.